The importance of alternative energy sources has increased over the last decades. One of these alternatives is solar energy and accordingly, the number of installations of PV (photo voltaic) systems has grown immensely.
PV systems (also: PV arrangements or PV assemblies) have been further developed in recent years to increase reliability and safety. Great efforts have for example been undertaken in order to avoid arcs in PV systems. However, how to deal with an arc once an arc occurs is a problem that still has not been solved satisfyingly.
Arcs may occur when a sufficiently high voltage is generated between two electrodes. The voltage may cause ionization of the gas (generally air) between the electrodes, such that a plasma may evolve and a current may flow between the electrodes. Such a plasma may heat up to several 1000° C. thereby causing the characteristically bright arc to appear. Such temperatures can cause damage to the system and in some cases even fire of the system or nearby installations.
There exist two different types of arcs in PV systems: parallel and serial arcs. Parallel arcs occur either between the positive and negative pole of the PV system or between one of the poles and ground. Serial arcs in contrast do not occur between two different lines but within one of the current conducting lines, i.e. between two parts of the same line. Serial arcs typically occur when a current in a line is disrupted, for example by opening a contact switch or in case of breaking plugs/cables or the like.
In order to reduce the impact of arcs countermeasures to break an arc have to be initiated as soon as possible. It is therefore of high importance that the presence of an arc is determined as fast and as reliable as possible.
U.S. Pat. No. 8,179,147 B2 discloses a method and apparatus for managing series and parallel DC arc faults in a DC circuit of PV systems. Such a PV system comprises a power stage with several branches connected in parallel where each branch includes a PV module (104) connected to a power converter (102). The power stage is connected to a load center (108) which is connected to an AC power grid.
Detection of an arc fault is done by analyzing a signature of a current signal and analyzing a signature of a voltage signal of the power converter and determining based on this analysis, whether an arc fault exists and what type of arc fault is present. In particular, an arc is determined by analyzing the current signal for a change in polarity or a rapid change in slope of the signal. If a potential arc fault is identified, the voltage signal is analyzed as well and then compared to the current signal analysis. If an arc fault match exists, the DC current polarity is utilized to differentiate whether a series or a parallel arc has occurred. Based on the determination of the presence and the type of the arc, further measures are initiated.
The known method and apparatus for detection of an arc is very complicated. Accordingly, it takes a lot of effort, different elements and time to analyze the different signals, to compare the results of the analysis and to determine the presence and the type of an arc.